Patent GB-495057-A (GB)
Title: Improvements in or relating to electric systems for giving air-raid warnings
IPC: G08B3/10
Filed: 1937-05-07
Abstract: 495,057. Electric control systems. STANDARD TELEPHONES & CABLES, Ltd., and BOWSHER, E. A. H. May 7, 1937, No. 13100. [Class 40 (i)] In a system for giving air raid warnings over an area served by an electric power network, a number of hooters distributed over the area are connected to the network singly or in groups by signals transmitted over the network. The signals for operating the hooters may also be effective to switch off lights supplied from the network. Figs. 1 and 2 show multi-service equipment at the control station, positive or negative impulses being transmitted over the power network 1, 2, 3, N in response to operation of keys KA .. KE, DP, LK. AL associated with a distributer W. A resistance R connected between the star-point of the network and the earthed neutral N is normally shortcircuited by the armature LA of a mechanically latched contactor OC, RC. Throwing a key in one or other direction operates relay CB or DC which opens the short-circuit and applies battery BY across resistance R to transmit current of one or other polarity. The key KB is used for giving air raid warnings without affecting the lights whilst key AL causes operation of the hooters and switching off of the lights. Keys DP, LK control the switching on and off of the lights and the other keys control additional services such as water heaters. Fig. 3 shows a synchronous switch at a receiving point for controlling the hooters. The lights are controlled by polarized relays connected to the network. To switch on the lights, key LK is operated to the right to connect relay PR to contact S of the distributer. The distributer motor M is started by key SK and maintained in operation for one revolution by cam contacts DNS and when arm W engages contact S, relay PR pulls up and relay CB is operated to transmit a positive pulse over the network and operate the polarized relays of the lights. At the receiver, Fig. 3, relay DN is operated and motor M is started over wiper W and maintained in operation over cam contacts CM. Since, however, no further pulses are transmitted the rotation of the synchronous switch is without effect. To operate the warning hooters without affecting the lights, key KB is operated to the right to mark one of the contacts 1 .. 10. Positive pulses are transmitted at the commencement of the cycle and at a point in the cycle corresponding to the marked contact to start the receiver switch and effect the operation of the relay S which switches on the hooters. The hooters are switched off by a pulse sent after a different time interval as determined by reverse operation of the key KB. The lights remain unaffected since the transmitted pulses are of the same polarity as the switching-on pulse of the lights. Halfnight lights are switched off by operation of the key LK to transmit a short negative pulse. All-night lights may be switched off by transmitting a long pulse by operation of key DP which marks segment D. When relay DN, Fig. 3, is operated by a long negative pulse relay WO is operated to prevent operation of the relay S. Air raid warnings may also be given by operation of key AL which causes transmission of a long negative pulse over contact A to switch the alarms on and off successively. The long pulse also switches off all the lights. The battery BY may be replaced by a rectifier which when shunted across the resistance R produces in the network a unidirectional current component of the A.C. supply current. Specification 472,431 is referred to.